A snatch block consists of a hanger attachable to a davit by a swivel and provided with a sheave exposed to enable a line to be brought into engagement with the sheave from one side thereof.
Snatch blocks are widely used by lobster fishermen to enable traps to be hauled by a trap hauler to the side of their boats, lifted from the water and then swung inboard. In practise, the line adjacent the pot is caught and manually pulled to provide enough line to enable it to be supported by the sheave and caught by the trap hauler. The sheaves of snatch blocks typically have a central portion of minimum diameter and frusto-conical side portions the larger, outer ends of which are free fits within fixed water shields. This design enables toggles, line splices, etc. to pass freely through the sheave.
A problem encountered in the use of snatch blocks arises from the fact that the position of a boat relative to the trap being hauled varies due to such factors as currents and the direction and strength of the wind. If the trap being hauled is directly abaft, the line runs substantially straight from the trap over the sheave to the trap hauler. If a trap is more or less ahead of the boat or more or less astern thereof, an angle exists in the line between that portion between the trap and the sheave and the portion extending from the sheave to the trap hauler.
Any such angle tends to cause the hanger to swing and the line to move from the central portion of the sheave onto one of the frusto-conical portions thereof and also to cause the swivelling of the hanger. If the angle between such line portions decreases to an extent such that the line is pulled against the adjacent fixed water shield not only is the line chafed but also, if the water shield is at the open side of the snatch block, the line may and not infrequently does escape from the sheave. When that happens, it is necessary to reposition the boat and again catch the line to enable the interrupted hauling to be continued.